Diiron Complexes with Rigid and Conjugated S-to-S Bridges for Electrocatalytic Reduction of CO₂

Electroreduction of CO₂ to value-added low-carbon chemicals is a promising way for carbon neutrality and CO₂ utilization. It was found that the diiron complex [(μ-bdt)Fe₂(CO)₆] (bdt = benzene-1,2-dithiolate) has high catalytic activity for electrocatalytic CO₂ reduction. To further study the effect of the S-to-S bridge on the catalytic performances of diiron complexes for electrochemical CO₂ reduction, four diiron complexes 1-4 with different rigid and conjugated S-to-S bridges were either selected or designed. The electrocatalytic studies showed that under optimal conditions, 2 with a 2,3-naphthalenedithiolato bridge exhibited the lowest catalytic onset potential (E_onset = -1.75 V vs Fc^+/0), while 4 with a diphenyl-1,2-vinylidene bridge displayed the highest catalytic activity (TOF_max = 295 s^-1), which is 1.5 times that of [(μ-bdt)Fe₂(CO)₆]. The controlled potential electrolysis experiments of 4 in 0.1 M MeOH/MeCN at -2.35 V vs Fc^+/0 gave a total faradaic yield close to 100%, with selectivities of 77%, 9%, and 14% for HCOOH, CO, and H₂, respectively. The mechanism for CO₂ reduction was studied using density functional theory, IR spectroelectrochemistry, and electrochemical methods. The results indicate that modifying the structure of the S-to-S bridge is an effective strategy to improve the catalytic performance of diiron complexes for electrocatalytic CO₂ reduction.